1. Field of the Invention
The present invention relates to a method and a circuit for quick charging of a secondary cell, such as a nickel-cadmium cell.
2. Description of the Related Art
A secondary cell means a rechargeable cell and is distinguished from an unrechargeable primary cell such as a dry cell. Primary cells include such dry cells as manganese cells, alkaline cells, and mercury cells, which are on the market and widely used for transistor radios, cameras, tape recorders, watches, and so on.
On the other hand, secondary cells are frequently used for communications terminals such as cordless phones and portable phones, in which when the voltage of the cell drops, it can be recharged and used again and again without exchanging it with a new one. Of the secondary cells, those most extensively used are nickel-cadmium cells.
In charging methods of a secondary cell, there are two kinds, i.e., the standard charging method and the quick charging method. The standard charging method is such that charges a cell with a current approximately one tenth of the cell capacity and taking a time of about 15 hours. In this method, since the charging current is small, no problem arises even if the charging time goes, to a certain degree, over the standard time. In this method, however, since it takes more than half a day before the cell can be used again after it has lost its capacity, effective use of the apparatus cannot be attained.
In the quick charging method, the cell is charged with a current about 1.5 times as large as the cell capacity and taking a time of about one hour. Since this method finishes the charging in one hour or so, effective use of the apparatus can be attained. However, since the cell is charged with a current larger than the cell capacity, it sometimes occurs that the cell is overcharged if the point of completion of the charging is not accurately detected and that, in the worst case, the cell is damaged. More specifically, if the charging operation is continued even after the cell is fully charged, the electrolyte is gasified and the cell is exploded.
There have been in practice two quick charging methods of secondary cell. One is such that measures the rate of change, i.e., the gradient, of charging voltage to charging time and the other is such that measures change of the surface temperature of the cell with charging time.
The method that measures the rate of change of charging voltage to charging time will be described below with reference to FIG. 1A and FIG. 1B. Having a cell 2 connected with a battery charger 4 and a voltmeter 6 as shown in FIG. 1A, the cell 2 is quickly charged while the voltage of the cell 2 is being measured. Supposing that the cell voltage is 1.2 V and the cell capacity is 500 mAH, charging the cell 2 is completed in about one hour with a current of 500 mA passed through the cell. In this charging method, with the gradient .DELTA.V of the charging voltage to the time measured, the charging must be stopped as soon as the charging voltage has reached the peak voltage and the gradient become -.DELTA.V as shown in FIG. 1B.
Now, the method to measure the surface temperature of a cell to detect completion of charging will be described with reference to FIG. 2A and FIG. 2B. In this method, as shown in FIG. 2A, a temperature sensor 8 is attached to the conductive casing as the negative electrode of the cell 2. The charging is stopped immediately after the surface temperature of the cell 2 has reached a specified charge completion temperature T.sub.0. If the charging is continued past the specified temperature T.sub.0, there is a danger that the cell will be broken. Since the surface temperature of the cell 2 at the time of completion of charging can change according to the ambient temperature, the specified temperature T.sub.0 must be adjusted according to the ambient temperature.
Whichever of the above described quick charging method is employed, charging is performed with a current larger than the cell capacity. Hence, unless the point of completion of charging is accurately detected, overcharging is caused, and this leads to such problems as occurrence of leakage of the electrolyte or gas, and if the overcharging is continued, the life of the cell is shortened or the capacity of it decreased.